Field of the Invention
The present invention relates to an image pickup optical system, and is suitable to an image pickup apparatus such as a still camera, a video camera, a digital still camera, and a monitor camera.
Description of the Related Art
Conventionally, there is a demand for an image pickup optical system which is easy for a capture at a near distance where a capturing magnification is approximately equal magnification. In general, in most of image pickup optical systems, variations in various aberrations, for example, spherical aberration and field curvature, are increased as the capturing magnification increases, and an optical performance is lowered. Conventionally, there is known an image pickup optical system which employs a so-called floating method in which a plurality of lens units are individually moved during focusing to correct the variation of various aberrations during focusing (Japanese Patent Application Laid-Open No. 2009-69414). With the use of the floating method, a variation in aberrations is reduced during focusing from infinity to a close distance, and a good optical performance is easily obtained even in close-distance capturing.
Japanese Patent Application Laid-Open No. 2009-69414 discloses a retrofocus lens which is configured by a first lens unit having a negative refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power which are disposed in an order from an object side to an image side. In Japanese Patent Application Laid-Open No. 2009-69414, the respective lens units are moved toward the object side such that an interval between adjacent lens units is changed during focusing from infinity to a close distance. In particular, the entire system is moved toward the object side and the second lens unit is moved by a moving amount different from the other lens units at the time of close-distance capturing, so that the variation in aberration at the time of close-distance capturing is reduced.
In most of the image pickup optical systems, various aberrations such as the spherical aberration are increased in close-distance capturing. Therefore, the floating method in which two or more lens units are moved during focusing is significantly effective in reducing the variation in aberration in close-distance capturing.
However, it is important to use an appropriate floating method and to appropriately set the entire lens configuration in order to reduce the variation in aberration and to obtain a high optical performance in the entire object distance during focusing. If these configurations are not appropriately set, the entire system is increased in size, and the variation in various aberrations is increased during focusing. It is very difficult to obtain a high optical performance over the entire object distance and the entire screen.
In particular, it is important to appropriately arrange an aperture stop in order to easily perform the close-distance capturing while downsizing the entire lens system. For example, if a distance from a lens surface closest to the image side to the aperture stop is long in the close-distance capturing, there is a need to increase a barrel diameter between the lens and an image plane in order to secure a peripheral light amount in the close-distance capturing, which leads to an increase in outer diameter of the lens. In addition, if the distance from the lens surface closest to the object side to the aperture stop is short, it is difficult to dispose a cam barrel necessary to secure a feeding amount of the lens unit which performs focusing in the close-distance capturing.